1. Field of the Invention
This invention relates to a method for forming an energy subtraction image wherein, from a plurality of radiation images, an energy subtraction image is formed which includes little noise and which has good image quality and can serve as an effective tool in, particularly, the efficient and accurate diagnosis of an illness. This invention also relates to an image smoothing method, which is to be used in the method for forming an energy subtraction image, and an apparatus for carrying out the image smoothing method.
2. Description of the Prior Art
Techniques for reading out a recorded radiation image in order to obtain an image signal, carrying out appropriate image processing on the image signal, and then reproducing a visible image by use of the processed image signal have heretofore been known in various fields. For example, as disclosed in Japanese Patent Publication No. 61(1986)-5193, an X-ray image is recorded on an X-ray film having a small gamma value chosen according to the type of image processing to be carried out, the X-ray image is read out from the X-ray film and converted into an electric signal, and the electric signal (image signal) is processed and then used for reproducing the X-ray image as a visible image on a copy photograph or the like. In this manner, a visible image having good image quality with high contrast, high sharpness, high graininess, or the like can be reproduced.
Also, when certain kinds of phosphors are exposed to radiation such as X-rays, .alpha.-rays, .beta.-rays, .gamma.-rays, cathode rays or ultraviolet rays, they store part of the energy of the radiation. Then, when the phosphor which has been exposed to the radiation is exposed to stimulating rays such as visible light, light is emitted by the phosphor in proportion to the amount of energy stored thereon during its exposure to the radiation. A phosphor exhibiting such properties is referred to as a stimulable phosphor. As disclosed in U.S. Pat. Nos. 4,258,264, 4,276,473, 4,315,318, 4,387,428, and Japanese Unexamined Patent Publication No. 56(1981)-11395, it has been proposed to use stimulable phosphors in radiation image recording and reproducing systems. Specifically, a sheet provided with a layer of the stimulable phosphor (hereinafter referred to as a stimulable phosphor sheet) is first exposed to radiation which has passed through an object, such as the human body. In this manner, a radiation image of the object is stored on the stimulable phosphor sheet. The stimulable phosphor sheet, on which the radiation image has been stored, is then scanned with stimulating rays, such as a laser beam, which cause it to emit light in proportion to the amount of energy stored during exposure to the radiation. The light emitted by the stimulable phosphor sheet, upon stimulation thereof, is photoelectrically detected and converted into an electric image signal. The image signal is then used during the reproduction of the radiation image of the object as a visible image on a recording material such as photographic film, on a display device such as a cathode ray tube (CRT), or the like.
Radiation image recording and reproducing systems which use stimulable phosphor sheets are advantageous over conventional radiography using silver halide photographic materials, in that images can be recorded even when the energy intensity of the radiation to which the stimulable phosphor sheet is exposed varies over a wide range. More specifically, since the amount of light which the stimulable phosphor sheet emits when being stimulated varies over a wide range and is proportional to the amount of energy stored thereon during its exposure to the radiation, it is possible to obtain an image having a desirable density regardless of the energy intensity of the radiation to which the stimulable phosphor sheet was exposed. In order for the desired image density to be obtained, an appropriate read-out gain is set when the emitted light is being detected and converted into an electric signal (image signal) to be used in the reproduction of a visible image on a recording material, such as photographic film, or on a display device, such as a CRT.
In the radiation image recording and reproducing systems wherein recording media, such as X-ray film or stimulable phosphor sheets are used, subtraction processing techniques for radiation images are often carried out on image signals detected from a plurality of radiation images of an object which have been recorded on the recording media.
With the subtraction processing techniques for radiation images, an image is obtained which corresponds to a difference between a plurality of radiation images of an object recorded under different conditions. Specifically, a plurality of the radiation images recorded under different conditions are read out at predetermined sampling intervals, and a plurality of image signals thus detected are converted into digital image signals which represent the radiation images. The image signal components of the digital image signals which represent the image information recorded at corresponding sampling points in the radiation images are then subtracted from each other. A difference signal is thereby obtained which represents the image of a specific structure or part of the object represented by the radiation images.
Basically, subtraction processing is carried out with either the so-called temporal (time difference) subtraction processing method or the so-called energy subtraction processing method. In the former method, in order for the image of a specific structure (for example, a blood vessel) of an object to be extracted from the image of the whole object, the image signal representing a radiation image obtained without injection of contrast media is subtracted from the image signal representing a radiation image in which the image of the specific structure (for example, a blood vessel) of the object is enhanced by the injection of contrast media. In the latter method, such characteristics are utilized that a specific structure of an object exhibits different levels of radiation absorptivity with respect to radiation with different energy levels. Specifically, an object is exposed to several kinds of radiation with different energy levels, and a plurality of radiation images are thereby obtained in which different images of a specific structure are embedded. Thereafter, the image signals representing the plurality of the radiation images are weighted appropriately and subjected to a subtraction process in order to extract the image of the specific structure. The applicant proposed novel energy subtraction processing methods using stimulable phosphor sheets in, for example, U.S. Pat. Nos. 4,855,598 and 4,896,037.
A plurality of radiation images, which are subjected to energy subtraction processing, will herein be referred to as the "original images". An image signal representing a subtraction image is obtained by subtracting the image signals representing the original images from each other. Therefore, the image signal representing the subtraction image has a lower signal-to-noise ratio (S/N ratio) than the image signals representing the original images. As a result, the problems occur in that the image quality of the subtraction image becomes worse than the image quality of the original images.
By way of example, energy subtraction processing is often carried out in the manner described below. Specifically, an object, such as the chest of a human body, which is constituted of soft tissues and bones, is exposed to several kinds of radiation with different energy levels, and a plurality of radiation images of the object are thereby obtained. The plurality of the radiation images are read out, and a plurality of image signals representing the radiation images are generated. Energy subtraction processing is then carried out on the plurality of the image signals. From the energy subtraction processing, a soft tissue image signal is obtained which represents a soft tissue image primarily composed of patterns of the soft tissues of the object. Alternatively, a bone image signal is obtained which represents a bone image primarily composed of patterns of the bones of the object. Thereafter, the soft tissue image is reproduced as a visible image from the soft tissue image signal, or the bone image is reproduced as a visible image from the bone image signal. In the soft tissue image, the patterns of the bones have been erased. Therefore, patterns, which were behind the bone patterns or were rendered imperceptible by the bone patterns in the original images, become more perceptible in the soft tissue image than in the original images. Also, in the bone image, the patterns of the soft tissues have been erased. Therefore, patterns, which were behind the soft tissue patterns or were rendered imperceptible by the soft tissue patterns in the original images, become more perceptible in the bone image than in the original images. Accordingly, a subtraction image can be obtained which is well matched to the purposes of diagnosis. However, because the soft tissue image and the bone image are obtained from the subtraction processing, the problems occur in that noise components have been emphasized in the soft tissue image and the bone image than in the original images. From this point of view, the image quality of the soft tissue image and the bone image could not heretofore been kept good.